Uranium-238, natural abundance

Uranium-235 Enrichment. The enrichment of uranium is expressed as the weight percent of in uranium. For natural uranium the enrichment level is 0.72%. Many appHcations of uranium requite enrichment levels above 0.72%, such as nuclear reactor fuel (56,57). Normally for lightwater nuclear reactors (LWR), the 0.72% natural abundance of is enriched to 2—5% (9,58). There are special cases such as materials-testing reactors, high flux isotope reactors, compact naval reactors, or nuclear weapons where enrichment of 96—97% is used. 

The only large-scale use of deuterium in industry is as a moderator, in the form of D2O, for nuclear reactors. Because of its favorable slowing-down properties and its small capture cross section for neutrons, deuterium moderation permits the use of uranium containing the natural abundance of uranium-235, thus avoiding an isotope enrichment step in the preparation of reactor fuel. Heavy water-moderated thermal neutron reactors fueled with uranium-233 and surrounded with a natural thorium blanket offer the prospect of successful fuel breeding, ie, production of greater amounts of (by neutron capture in thorium) than are consumed by nuclear fission in the operation of the reactor. The advantages of heavy water-moderated reactors are difficult to assess. 

As the parent of actinium in this series it was named protoactinium, shortened in 1949 to protactinium. Because of its low natural abundance its chemistry was obscure until 1960 when A. G. Maddock and co-workers at the UK Atomic Energy Authority worked up about 130g from 60 tons of sludge which had accumulated during the extraction of uranium from UO2 ores. It is from this sample, distributed to numerous laboratories throughout the world, that the bulk of our knowledge of the element s chemistry was gleaned. 

The sequences of radioactive decays that lead to lead are well-known and the rates of decay have been carefully measured. We shall consider the sequence based upon the relatively slow decomposition of the most abundant uranium isotope, mass 238 (natural abundance, 99%) ... 

Uranium is the fuel of nuclear reactors. The most important of its minerals is pitchblende, U02 (Fig. 17.28), much of which is obtained from strip mines in New Mexico and Wyoming. Uranium is refined to reduce the ore to the metal and to enrich it that is, to increase the abundance of a specific isotope—in this case, uranium-235. The natural abundance of uranium-235 is about 0.7% for use in a nuclear reactor, this fraction must be increased to about 3%. 

Occurrence. The natural abundance of Tc is negligibly small. Technetium is a by-product of the nuclear industry and it is a product of the uranium decay. 

We now appreciate the reasons for the high cost of enriched uranium. The single stage separation factors are low, as is the natural abundance. This leads inexorably... 

S is the selectivity of photon absorption under the particular experimental conditions, and 8 is the relative abundance of the desired isotope. Equation 8.19 shows, for example, that S values on the order of 103 are required before more than about 10% of photons are used to excite D in natural abundance H/D mixtures (8 1.5 x 10-4). The selectivity required for uranium separation is less because > 8(D). 

Low-Level Waste. Low-level wastes are further divided into categories of special nuclear material, source material, and byproduct material, depending on the isotopes contained. Special nuclear material refers to uranium 233, plutonium 239, and uranium containing more than the natural abundance of uranium 235. Source material refers to materials containing 0.05 percent or more of thorium or uranium in any physical or chemical form except that covered under special nuclear material. By-product materials consist of all other radioactive materials including fission and activation products. 

Natural abundance uranium contains 99.3% 92U238 and 0.7% 92U235. Depleted uranium is a term used to describe uranium metal that contains 0.2% of the 235 isotope. It is also referred to as D-38. 

We should not leave our discussion of nuclear reactors without mentioning the Oklo phenomenon. In 1972, French scientists analyzing uranium ore from the Oklo uranium mine in Gabon found ore that was depleted in 235U. Further investigation showed the presence of high abundances of certain Nd isotopes, which are formed as fission products. The relative isotopic abundances of these isotopes were very different from natural abundance patterns. The conclusion was that a natural uranium chain reaction had occurred 1.8 billion years ago. 

U has a half-life of 4.47 X 109 years. Use the Radioactive Decay activity (eChapter 22.3) to determine the half-life of 235U. Considering that the atomic mass of uranium is greater than 238, which isotope do you expect to be more abundant in nature Comment on the apparent relationship between half-life and natural abundance of these two isotopes. 

Assume that you have a sample of uranium hexafluoride with the natural abundance (0.7%) of 235U, and that you want to use gaseous effusion to separate the isotopes. 

Having purified the uranium, it is then treated to separate the U and U isotopes for nuclear fuel purposes (any uranium compounds purchased commercially are already depleted U). In practice, nuclear fuel requires enrichment from the natural abundance of 0.71 %... 

The natural abundance of in uranium is 0.79 atom %. If a sample of uranium is enriched to 3 at. % and then is stored in salt mines under the ground, how long will it take the sample to reach the namral abundance level of(assuming no other processes form this is not the case if is present since it can decay to form " U) The half-life of " U is 7.13 X 10 years. 

Uranium with isotopic abundances different from that of natural uranium is the primary signature for HEU production activities. In any separation technology some enriched uranium will inevitably be released to the environment. Environmental samples taken at or near an enrichment facility can contain some of the enriched material altering the uranium isotopic abundance. Analysis of samples of vegetation, water and soil for uranium isotopic content using a sensitive analytical technique, such as thermal ionization mass spectrometry is recommended as the primary technique for the detection of HEU production. 

TABLE 13.1 Typical natural abundances of uranium and thorium in the Earth s crust... 

The natural abundance of the uranium-235 needed for weapons and reactors is only about 0.7 percent of uranium found naturally urani-um-238 makes up most of the rest. During World War II, scientists separated out the uranium-235 by a process in which all the uranium was converted into uranium hexafluoride gas (UFg). The separation was possible because the gas molecules of one of the isotopes diffuse faster than those of the other. Which isotope s molecules will diffuse faster Explain. 

Heat generation takes place in the reactor core of a nuclear plant (Figure 23.15). The core contains the fuel rods, which consist of fuel enclosed in tubes of a corrosion-resistant zirconium alloy. The fuel is uranium (IV) oxide (UO2) that has been enriched from 0.7% the natural abundance of this fissionable isotope, to the 3% to 4% required to sustain a chain reaction. Sandwiched between the fuel rods are movable control rods made of cadmium or boron (or, in nuclear submarines, hafnium), substances that absorb neutrons very efficiently. 

Polonium is found in the earth s crust at exceedingly low levels its natural abundance is only 2 x 10 ° milligrams per kilogram. Polonium is produced in pitchblende when the bismuth isotope °Bi, which has a half-fife of five days, decays into °Po. Approximately 100 micrograms of polonium are found in 1 ton of uranium ore. Polonium can also be produced by bombarding ° Bi with neutrons to form °Bi, which in turn decays into °Po. 

Determination of a fission track age requires several further experimental steps to measure the uranium concentration. The uranium concentration is not measured directly, but a second set of fission tracks is created artificially in the sample by a thermal neutron irradiation. This irradiation induces fission in a tiny fraction of the atoms, which are present in a constant ratio to U in natural uranium. Knowing the total neutron fluence received during irradiation, the number of induced tracks provides a measure of the uranium concentration of the grain. Because the induced tracks are derived from a different isotope of uranium than the spontaneous tracks an important consideration in fission track dating is the assumption that the isotopic ratio of the two major isotopes of uranium, and is constant in nature. With the notable exception of the unique natural nuclear reactors of Oklo in Gabon (Bros et al. 1998), where this isotopic ratio is disturbed, this is a very safe assumption. Numerous measurements have shown that and are always present in their natural abundances of 0.73% and 99.27%, respectively. 

The uranium decay series consist of a group of nuclides that, when their mass munber is divided by 4, have a remainder of 2 (the 4 + 2 series). The parent of this series is with a natural abundance of 99.3% it undergoes a-decay with a half-life of 4.46 X lO y. The stable end product of the uranium series is ° Pb, which is reached after 8 a- and 6 /3-decay steps. 

The 30% savings in natural uranium for LWR and similar reactors and the hundredfold energy resource expansion for FBRs when reprocessing spent iiiel, has already been discussed in the previous section. The economic advantage of reprocessing depends on the cost and availability of natural ("yellow cake") uranium, on enrichment and other front end activities, and on the prevailing energy price (mainly based on fossil fuels). At present, cheap uranium is abundant. 

The table below shows the percentage of natural abundance of each natural uranium isotope, and their respective half-lives. 

Different isotopes of an element have different natural abundances. For example, 99.3% of naturally occurring uranium is uranium-238, 0.7% is uranium-235, and only a trace is uranium-234. Different isotopes of an element also have different stabilities. Indeed, the nuclear properties of any given isotope depend on the number of protons and neutrons in its nucleus. 

For the development of nuclear energy for military purposes in World War II, the Manhattan Project of the U.S. Army Engineers Corps required large quantities of D.O, highly enriched and the fissionable isotope of uranium, 235y most difficult task was the production of kilograms of 90% U from the natural abundance of 0.7%. Many processes were... 

Almost 60,000 moles of natural abundance uranium flow through the cascade, at the feed stage for each mole of 90% 235u withdrawn. 

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